1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus provided with a separation charge-eliminator which separates a recording medium from an image bearing member after a developer image formed on the image bearing member is transferred onto the recording medium.
2. Description of the Related Art
The electrophotographic image forming apparatus includes an image bearing member, a charging device which charges the image bearing member, an exposure device which forms an electrostatic image on the image bearing member, a developing device which develops the electrostatic image by using a developer (toner), and a transferring device which transfers a toner image formed on the image bearing member by the developing device onto the recording medium.
A transfer roller being the transferring device is disposed in contact with the image bearing member (photosensitive drum) and forms a transferring nip in a contact region. A transfer voltage having a polarity reverse to the toner image is applied to the transfer roller to thereby transfer the toner image on the photosensitive drum onto the recording medium. However, the polarity of the potential of a surface of the photosensitive drum is a polarity reverse to the polarity of the recording medium after the transferring, and hence when the recording medium passes through the transferring nip in which the transfer roller is in contact with the photosensitive drum, the recording medium may be attracted electrostatically to the photosensitive drum. Accordingly, the recording medium is electrostatically attracted to the photosensitive drum, which causes a problem of image quality deterioration on the recording medium or a conveyance failure (jam) of the recording medium.
In order to solve this problem, up to now, it is proposed that a separation charge-eliminator which eliminates charges from the recording medium immediately after the recording medium passes the transferring nip be provided in a position in proximity to the transferring device. A separation voltage having the polarity reverse to the polarity of the recording medium after the transferring is applied to the separation charge-eliminator.
In the image forming apparatus that employs a method of separating the recording medium from the image bearing member as described above, a stain on the separation charge-eliminator is one of failures that arise when an interior of the apparatus is stained with paper dust or flying toner. When the paper dust or the toner adheres to and stains the separation charge-eliminator, the electric resistance thereof increases, and in a case where the separation voltage is a constant voltage, the separation charge-eliminating current decreases. Accordingly, the separation charge-eliminator becomes less effective in charge elimination, and hence the recording medium is kept from being separated from the photosensitive drum, which leads to a fear that a recording medium jam may occur or the image quality may deteriorate.
Therefore, an image forming apparatus described in Japanese Patent Application Laid-Open No. 2005-241947 applies the transfer voltage to the transferring device, and detects the current value of a current flowing from the transferring device into the separation charge-eliminator by discharge. It is proposed therein that the separation voltage to be applied to the separation charge-eliminator be controlled depending on the detected current value to thereby alleviate the image quality deterioration or a jam occurrence.
In recent years, various kinds of recording media used for the image forming apparatus are distributed, and some recording media cause a large amount of paper dust when the recording medium is transported. In the case of using a large number of recording media that cause a large amount of paper dust, the paper dust is piled up in a periphery of the separation charge-eliminator so as to cover the separation charge-eliminator at an early stage after the start of use of the image forming apparatus.
By using the technology described in Japanese Patent Application Laid-Open No. 2005-241947, the stain such as the piled-up paper dust is detected only in a direction in which the current flows into the separation charge-eliminator from the transferring device located on an upstream side of the separation charge-eliminator in a recording-medium conveying direction. This leads to a fear that it may not be possible to detect the paper dust piled up in a periphery of a distal end of the separation charge-eliminator disposed opposite to the image bearing member in a direction from the separation charge-eliminator to the image bearing member.